Generally, referring to FIG. 8, a conventional knife sharpener comprises a main body (70), a suction cup (71), an operating handle (72) and an upper cover (73). The main body (70) has a through hole penetrating through a central portion thereof, the suction cup (71) is configured to create a vacuum zone thus adhering to a flat surface. A rod penetrating through the through hole of the main body (70) is pivotally connected to the operating handle (72). The operating handle (72) comprises a protruding portion (721) located at a pivot point between the rod and the operating handle (72), and a lever (722) is formed at another end of the operating handle (72). A lid (73) covers on a top portion of the main body (70), and a standing plate protruding from a top surface of the lid (73) has a V-shaped notch which is configured to receive a sharpening stone (731) thereon. In actual application, a user can hold the suction cup (71) against a flat surface, and then pulls the operating handle (72) to bear the protruding column (721) against the main body (70) such that a vacuum zone is formed between the flat surface and the suction cup (71), and also the main body (70) is configured to press an outer edge of the suction cup (71) thus allowing the suction cup (71) to firmly adhere to the flat surface. As a result, a knife is configured to be moved forth and back on the sharpening stone (731) of the knife sharpener to achieve sharpening effect.
However, the conventional knife sharpener has following disadvantages: since the conventional knife sharpener can only provide a fixed sharpening angle, it cannot be widely used in all kinds of knives because different knives may have different specific sharpening angles, which reduces its practicability. Therefore, there remains a need for a new and improved design for a knife sharpener to overcome the problems presented above.